The present invention relates to a method and apparatus for measuring characteristics of materials relating to the permeability of the material. More particularly, the invention relates specifically to a method for measuring the diffusion coefficient, the solubility coefficient, and the permeability coefficient of a sample barrier material.
Certain apparatus for measuring the oxygen flow rate through membrane barrier materials are known in the prior art. For example, U.S. Pat. No. 3,618,361, issued Nov. 9, 1971, discloses an early system for measuring the gas permeability of a film. Similarly, U.S. Pat. No. 3,590,634 issued Jul. 6, 1971, discloses another instrument for measuring permeation rates through a membrane U.S. Pat. No. 4,464,927 issued Aug. 14, 1984, discloses an apparatus for measuring gas transmission through films in multiple test cells. All of these devices operate in conjunction with an oxygen detector which typically provides an electrochemical transformation in response to the presence of oxygen. One such oxygen detector is disclosed in U.S. Pat. No. 3,223,597, issued Dec. 14, 1965, and another form of oxygen detector is disclosed in U.S. Pat. No. 4,085,024, issued Apr. 18, 1978. All of these earlier patents, and a considerable number of more recent patents, utilize a test cell setup in conjunction with an oxygen detector to derive an electrical signal which is representative of the amount of oxygen within a given chamber of the test cell. A sample of the barrier material undergoing testing is typically clamped within the test cell to form two chambers, wherein one chamber is initially free of oxygen and filled with a neutral gas such as nitrogen, and the other chamber is initially saturated with oxygen. Before proceeding with these initial conditions, it is first necessary to outgas all oxygen from the material sample undergoing tests. Outgassing is accomplished by flowing a neutral gas such as nitrogen through both chambers described above, monitoring the test gas for oxygen content until it appears that the oxygen content has become depleted to zero, or near zero, and then proceeding with the initial Conditions described above. The test process requires that the neutral test gas flow be monitored until the oxygen concentration in the test gas reaches a steady state level, which can require many hours of operation. In general, the amount of time required for such a test is directly related to the permeability coefficient of the material and inversely related to the material thickness. The permeability coefficient is directly related to temperature and, to a lesser extent, pressure. The objective of tests of this type is to measure the amount of oxygen which permeates through the test membrane under steady state conditions, and the oxygen measurements are typically made by devices which are disclosed in the foregoing prior art patents.
The large majority of permeation measurements now being made are in terms of the amount of gas permeating a given sample. This may be a container or an essentially flat sample. The answers are given in terms of the volume or weight of a gas permeating the sample in a given time. In the case of a container, this becomes the volume or weight of gas per time per container. In the case of a flat sample, it is the volume or weight of a gas per time per unit area. These answers are obtained and refer to the conditions of the test. In a formal way, these are not permeation values but are transmission rate values for that gas, through that sample under the specific test conditions.
For example:
Sample--10 mil flat film (PET) PA1 Transmission Rate for O.sub.2 : ##EQU1## Pressure, 30.degree. C. and less than 5% RH.
The definition of the permeation rate for a film (in the same units) is referred to a standard temperature and pressure (STP) (760 mm Hg, 0.degree. C.) for a 1 mil film. The amount of gas being transferred is roughly inversely proportional to the film thickness. At the test conditions, a 1 mil film would then transmit ten times as much O.sub.2 as a 10 mil sample. ##EQU2## Correction to 0.degree. C. would then result in the formally defined permeation rate.
Basically, the permeation of gas through a material results from the inherent physical characteristics of the material. These characteristics have been formally defined in all of the literature in the field for the last 30 years. These characteristics are: the solubility of the material to the gas of interest in the material and the rate of diffusion of the gas through the material. The solubility coefficient "S" defines the volume of gas which will dissolve in a like volume of the material; i.e.(cm.sup.3 /cm.sup.3); and the diffusion coefficient "D" denotes the rate at which the gas moves through the material; i.e. (cm.sup.2 /sec). The product of the solubility coefficient "S" and the diffusion coefficient "D" is called the permeability coefficient "P"; and in this case, the units are [(cm.sup.3 /sec)/(cm.sup.2 /cm)], interpreted as "cubic centimeters of gas per square centimeter of area per second, per centimeter of thickness of material." The permeability coefficient "P" is related to the actual material permeability P.sub.actual ; i.e., the transmission rate of the gas through a given sample of material; by a simple conversion factor, which converts "cubic centimeters per second per centimeter of thickness" to a standard in terms of "cubic centimeters per day per 10 millimeter of thickness and per square meter of area," wherein the conversion factor is EQU P.sub.actual =P/(2.94.times.10.sup.-12).
As noted above, this information is well known. All aspects have been reviewed for years in the literature on the subject of permeation. The background is necessary, however, to follow the new method of measurement of the transmission of gas through a material.
A theoretical analysis of absorption and description of gases into materials is given by J. Frank in his book Mathematics of Diffusion, 2nd Edition, Clarendon Press, Oxford, England (1975).
The most used present method of measurement today is termed isostatic. This refers to the case in which a sample is mounted in such a way that one side of the sample is exposed to the gas of interest. The other side is isolated at zero, or extremely low levels, of that gas. In this way, the gas permeating the sample can be measured as a function of time.
Usually the film is first outgassed by flowing a neutral gas over both sides of the sample. Then the permeant gas is made to flow on one side, while the neutral gas flows on the other side through a sensor which is responsive to very small concentrations of the permeant gas. The final answer is obtained by waiting until the permeant gas concentration on the sensor side reaches a steady state value which is indicative of a steady-state permeation through the material sample. Typically, the time required for waiting until a steady-state permeation condition exists is usually quite long for even moderately good barriers. For instance, a PET film, 10 mil (10.sup.-2 inches) thick, at 30.degree. C., has a permeation (transmission) value of approximately 7.5(cm.sup.3 /M.sup.2)/day; i.e., 7.5 cubic centimeters per meter square per day. Many barriers today yield permeation measurements less than one-tenth of this value. With such materials, the preparatory outgassing of a 10 mil sample will take about 21 hours, and the permeation measurement requires about 29 hours.
It would be extremely desirable if the amount of time required for making valid permeation measurements could be significantly reduced. The equipment required for making such measurements is fairly expensive and complex; and therefore, the measurement of a single sample of material can require the exclusive use of a station having such equipment for a period of several days. If a significant number of samples require measurement, the number of test stations set up with the necessary equipment for such measurements must be multiplied to fit the testing schedule. Therefore, any modification through the overall process which can be made by way of shortening the total test time will be of great advantage and significance in the field.